If you have a gravity fed, open-vented heating system with a feed & expansion tank located in your loft, flushing the system is a simple but effective way to potentially increase its efficiency, maybe lower your running costs & improve the system’s lifetime. It’s also useful to flush the system if you’re looking at replacing radiators or making major pipework changes – not much point in replacing dirty radiators with new if you don’t clean the system.

Pressurised systems require extra steps and can be more difficult – do not use the instructions below as for a start, you won’t have a F&E tank to work with.

Over time & with normal usage, all heating systems build up a degree of “sludge” (iron oxide & dirt from water), which can restrict water circulation and cause your system to become more inefficient over time. Symptoms of a sludged system can include being slow to heat up, uneven heating of radiators, black or very dirty water when bleeding radiators along with many more issues. It can also accelerate failure of key system components such as pumps; simply because they have to work harder than they should to move the water around the system’s pipework.

The most heavily affected systems might require more serious intervention/treatment in the form of a “power flush” (a process which essentially force-circulates water & cleaning chemicals around the system with more force than is possible via other means) but for many other systems, a DIY flush may be sufficient to keep everything running in top condition.

There are many guides already available on the web that cover heating flushes in great detail, so I’m not going to go into much depth in this post. I have however summarised a few key steps below so that there’s hopefully enough detail for you to work out if this is something you’re confident to undertake and get you started.

Before you start, check that you have:

Access to the system’s F&E tank, and the ability to control water flow into the tank.

Access to a foul water drain (or toilet) within hosepipe reach of the F&E tank and a convenient drain point.

Access to a drain point/valve on your heating system

Sufficient length of garden hosepipe or similar to reach from the F&E tank to the nearby drain/toilet, and from the system drain point to a drain/toilet. These won’t be needed at the same time, so the same length of hose might be usable for both.

You will also need to buy:

Cleaning the system is a three step process (unless you’re powerflushing), as you need to flush & fill the system with the cleaning product; leave everything to circulate for a while and then flush/refill. It can take some time to carry out each flush/fill cycle depending on the size of your system, so ensure you have plenty of time and won’t need to abandon the job part-way through. The ideal moment to start this process would be probably be if you needed to take a radiator off for any reason or otherwise needed to drain the system down – as this would effectively empty out whatever’s currently in the system leaving it clear for you to introduce the cleaning products.

Assuming you’re not doing any other work on the system and that it’s currently full of water / working normally, we will need to drain/flush it through with clean water to clean out as much of the current sludge / dirty water as we can to start with. There’s not much point adding cleaning product to system full of dirty water as this will limit the product’s potential and will just mean that the process is not as efficient or as useful as it could otherwise be.

First things first – get a clean start

Before doing anything else however, go and have a look in the system’s Feed & Expansion (F&E) tank. If you’ve never properly flushed the system or cleaned the tank, I’d imagine you’ll find that it’s full of sludge. This usually takes the form of brown gunk on the tank’s walls, and can look like a thick layer of sand all over the tank’s base.

As a quick example of what you may be looking at, I don’t think our system’s F&E tank has never been properly cleaned out – despite all the work and changes that have been made to our system over the years since buying the property. At a rough guess, this is what the tank can look like after 20+ years of inattention…

It’s important if this is the case for your system to take the time now to clean out the F&E tank, removing the sludge BEFORE starting to drain down or flush the system. Otherwise, all you’ll do is disturb the gunk in your F&E tank by the act of it automatically refilling as its water level reduces; caused of course by the now dirty water being drawn into the system while you’re trying to flush it. This will just result in even more sludge entering the system, which will add to whatever’s in there already and limit the benefit of any further work.

Fortunately, cleaning the F&E tank is extremely simple to do and if you’ve ever cleaned out a fish tank or something similar using a hosepipe syphon as a vacuum cleaner… then the same process will work a treat here.

With the system switched off (mainly to avoid any unexpected discharge of hot water from the vent pipework!), the first step is to suitably arrange your hose pipe so that the outlet end is secured into whatever drainage point you identified. This is important – as once water’s flowing, you don’t want to be worrying about the hose coming loose… Water from the tank is going to be dirty and full of all the gunk that’s currently stuck to the tank – would be useful to dispose of it cleanly! We were using a nearby bathroom/toilet, so were able to secure the hose under the toilet seat. The other end of the hose needs to be taken to whereever the F&E tank is located, and temporarily secured so that it’s in the tank and under the surface of the water. Next, we need to get the water flowing. There are countless ways to achieve this from using some sort of specialised suction pump through to simply getting some water into the hose (hint – try using the mains water feed into your F&E tank) and letting gravity take over to applying a vacuum or suction to the outlet end of the pipe. Take your pick! Obviously, I cannot recommend the old-fashioned approach of sucking on the hose to generate suction; the water in the F&E tank is probably filthy and you really don’t want to be running even the slightest risk of ingesting any of it.

Once you’ve got suction going and are sure the hosepipe is secure in whatever you’re using for a drain, head back up to the F&E tank and temporarily stop the mains water feed to the tank while you use the hose to start emptying out the water currently in it. You can do this by either switching off a handy valve if you have one on the feed pipework or by simply holding the ballcock valve closed while water drains out.

Once you can actually see the bottom of the tank, use the hosepipe to start removing gunk from the tank. The easiest way is to simply use your hands (with some disposable gloves on if you prefer) to slosh the water around and agitate the gunk at the bottom of the tank – sucking it up with the hosepipe. Don’t let the tank empty completely as once your hose comes out of the water you’ll have to start again in terms of getting things flowing.

Keep going with this until you’ve got the gunk off the sides and bottom of the tank, and have flushed it all through with clean water a couple of times – ending up with a clean-ish tank full of clean water!

Once you’re happy with the tank (don’t worry about getting it perfectly shiny & new-looking… that’s not likely to happen unless you replace it), carefully take the hose out of the tank; allowing any remaining water to drain. Check that it’s automatically refilled with clean water, with the ball valve operating correctly to shut off the water feed once full.

Next, find a drain point & get connected

With the preparation work completed and a clean F&E tank ready to go, switch the system on and turn up its thermostat so that the pump & boiler run and get some circulation going around the pipework. Find a convenient drain point and connect up your hosepipe, again ensuring that the outlet end is sufficiently secured in whatever you’re using as a drain as you don’t want to end up with a system-full of dirty water all over the floor.

By this point, the system’s hopefully warmed up a little so switch it off for now and open up the drain point’s valve. If it’s not been used for a while, you’ll probably need to apply some force to open the valve; especially if it’s been painted over a few times since last used. The valve might also leak a little – we found a small paint tray to be about the ideal size to sit under the valve & catch any drips.

With the valve open, you should hopefully now have a steady flow of warm dirty water out of the system with corresponding flow of clean water into the system from the F&E tank. You shouldn’t need to open bleed valves to get water flowing (unlike if you’re draining the system down to empty) as your F&E tank should be able to continue replenishing water as fast as it drains out. Assuming the water’s brown or black, leave it running until it starts to clear. Depending on the size of system (number of radiators / amount of pipework etc), this might take some time. Once the water clears, close the drain valve and fire up the system. Leave it to heat up and circulate the water for 15-20 minutes, before switching off and reopening the drain valve. With a little luck the water will still be running clear – if not, leave it to flush again until it does.

Once the water is running clear, head back up to the F&E tank and close the ball valve so that the tank begins to empty. Once it’s down to about a third full, pour in your bottle of system cleaner (eg Fernox Central Heating Cleaner F3) and wait for the majority of remaining water to drain in to the system. Before the water level drops below the feed pipework to the system, re-open the ball valve and let the tank start to refill. Quickly head back down to the drain point, and close the valve to end the flush process.

At this point, you should have a central heating system that’s full of relatively clean water & cleaning chemicals. Disconnect your hosepipe and leave the system to run as normal for a week or so, circulating the cleaning chemicals around the pipework & all your radiators.

Finally…

After a week of normal usage, repeat the process above for flushing the system – starting after the system’s been able to heat up to normal temperature. This time, you’re flushing out the cleaning chemicals along with whatever sludge (suspended in the water) it’s been able to dislodge and again let the water run until clear. Once you’re happy that you have clean water running out, close the F&E tank’s ball valve and pour in your bottle of inhibitor (eg: Fernox Central Heating Protector F1).

With the inhibitor in the system, leave the F&E tank to refill and close your drain point’s valve.

Fire everything up, and let the system heat up to temperature. Check all your radiators are hot all over, and are not cold at the top. Its worth then working your way around and quickly bleeding them after a flush just to ensure no air has been caught somewhere.

Hopefully you’ll now have a heating system that’s working more efficiently, heats up quicker and might even last a little longer – along with saving yourself some cash by doing the work yourself.

Obviously, if you’re not confident in your ability to follow the steps above & deal with any problems should they occur, do not attempt to flush your central heating. Book a heating engineer or plumber to do it for you. I do not take any responsibility for anything that goes wrong, or right, should you choose to follow the notes above. You do so ENTIRELY at your own risk.

great article!! really glad I found it – one question though, in my case the same system is my heating AND hot water, so surely if I leave the cleaning chemicals in for a few days I can’t use my hot water, as surely it comes from same tank? so my bath water etc would then contain the chemicals?

Thank you both for your replies – my tank in loft has the expansion? bit on top of main tank i.e. it is one tank split into two I assume – so I assume this is same principle as you explain and as per diagram?

Okay, now I am confused then – as original reply mentioned a tank heating the water. But the only tank I have is this one I described. There must be some other tank or something I have not noticed – will have another look in the loft, is it a tank I am looking for, or just a plastic container with a lid? – Thanks for replying

The Fortic tank is a combined hot storage & cold water cistern from looking at details online. The smaller tank on the top fills the lower larger tank, which will be connected to your heating system along with probably having an electric immersion heater fitted?

It’s not overly clear what pipework runs to the hanging tank from your pics but that is probably the heating feed/expansion tank. Does it have an outlet pipe connected into your heating pipework somewhere, a mains cold feed supply with a ballcock and an open (vent) pipe from your heating system, perhaps hooked over the top so it vents into the tank?

Could be awkward to inspect or do anything with it though – ideally need to be able to see into it to establish the state it’s in / clean etc!

Hi Tim, yes that hanging tank has a ballcock connected to cold feed, a vent pipe hooked over top and another pipe from bottom going into mass of pipes for central heating – so this must be the right tanks

Location is not good as nearly impossible to get to easily – have managed to get a photo though which I have emailed you, looks dirty, but cleaning it out could be difficult due to location

I have been through this process about 4 weeks ago. In addition I flushed every radiator by disconnecting each from system and cleaning it under mains pressure by moving it to rear garden. Unfortunately my two biggest double radiators in conservatory are ice cold. I think my problem is sludge in the pipes under the floor boards on the ground floor, since my heating water drain cock is higher then the pipes under the floor boards. I have a very expensive very nicely fitted carpet throughout the house. At my age and small pension, I can not afford to disturb the carpet. Question any advice about cleaning the under floor 22mm pipes please. Look forward towards your kind response.

Hi there, I don’t profess to be a plumber but it does sound like your pipes are blocked in some way or potentially the valves could be stuck closed? Simple way to test that theory would probably be to take the problem radiator off / disconnect it, and then open each valve in turn to see if you can get any water flow through it. TRV valves can get stuck closed too – try taking the control head off the valve and ensure the pin operates properly. Hope this is of some use!

Hi Tim, I recently had to go through this whole process. (Clearing sludge) my system was renewed 6 years ago, but they did not clean out the F & E tank ?? so, not realising this I drained system then put the Fernox into tank, unaware of all the crap inside it. Needless to say all that crap then went into the system ??? So now starting at the begining again…..flushed etc, i then put my 2nd fernox via radiator (after cleaning out the tank of course) Let the whole thing run a few hours checked the tank What was clean water was now brown water. presumably from the safety vent pipe from boiler?? is that normal? was it drawing from around the boiler ? So frustrating ..but getting there slowly. interested that you first off didnt find it necessary to open any rad bleed point to flush out system ??

Sorry Max – just seen this! I’d imagine all the stuff in the tank would have indeed flushed through into the system if it wasn’t cleared first. More flushing would probably work to help clear that one! No idea unfortunately why you’d end up with brown water in the tank unless perhaps the system is overpumping or regularly venting into it? Might be worth watching the vent while the system’s running to see if it’s dumping water in there.

I don’t recall needing to open a bleed point however – possibly because I didn’t try and drain it; just flush through with a constant supply of water flowing in from the f/e tank and effectively out of a drain point.

Hi I recently cleaned the system out but only did it once didn’t reflush after chemicals 1st question is that ok? Also I have an open vented system and to get more pressure to some still cold radiators have put flexi pipe to top up pressure from the mains to boiler is that ok?

Hi Wendy, what chemicals did you leave in? If it was the cleaner, you might be better off to check with the manufacturer’s instructions as to how long it can be left in a system. Otherwise all the gunk it’s managed to dissolve/release will still be in the system. In terms of the open system & more pressure, good question. I’m not a plumber but if it’s open vented, that is probably not going to solve your problem as it’s not a pressurised system.

If you have TRV valves on the problem radiators, it’s worth removing the thermostatic heads and checking whether the valves are functioning correctly; there should be a pin visible underneath the head that moves freely up and down.. if it’s stuck and can’t be freed with a few gentle taps then you may need to replace the valve. If the pin moves OK, then try running the system without the thermostatic heads and see if the radiators warm up.

If they still remain cold, probably best to then check whether you’ve got decent water flow through the pipework to them. Can verify this by turning off both the valves, placing a bowl underneath one of them to catch the water and then loosening the connection from one valve to the radiator. Drain out the water from the radiator and then gently open the valve.. all being well you should be getting a decent flow through the valve. Close it & repeat with the other. If the flow’s OK, then maybe flush the radiator through outside to remove any persistent sludge and re-fit. If the flow’s not OK, then either the valves are not opening properly or there’s a problem / blockage / airlock in the pipework feeding that radiator somewhere. Might be easiest to get someone in to powerflush the system at that point unless you fancy tackling it yourself..

Hi Genny, from the sound of things either the ballcock isn’t fully closing the water feed or perhaps your system’s overpumping. If it happens when your system is off, then it’s probably the ballcock; if it only happens when the system’s running, then may be worth checking for air in radiators at the highest point of your system and potentially turning your pump’s speed down a notch.

Great article – thank you. will know (or let my husband to read this) what to do next time. We have a new boiler installed two months ago, a chemical flush of the system was included in the quote. Unfortunately, my dear husband did drill through one of the radiator pipes recently and, on top of ruining our carpet, we have noticed that the water was very very dirty. Does it mean that the engineer didnt do his job correctly, as i would assume the water to be clear-ish, or the colour of the water has nothing to do with the supposed flush? Do I complain to the installers? we have spend a small fortune on the new boiler and if the job hasnt been done properly, i would not be impressed… Many thanks in advance!!!

It’s really hard to comment further on that one to be honest. Chemical cleans are not really any different to the diy approach I’ve described – in that you introduce a cleaner chemical to the system, run for a while and then flush everything out. The results depend entirely on the state of the system and how much effort / time you put into flushing. No heating system will be pristinely clean or have clear water circulating around it though. If you’re concerned suggest speaking to whoever installed the boiler, attempting a couple of DIY cleans or maybe getting it powerflushed. If everything works and the radiators heat up quickly however, would suggest not worrying about it.

I have done this a few times in the past and had forgotten some of the tricks.

I was thinking that one trick for filling a hose with water prior so as to act as a syphon would be simply to put the hose in the bath to fill the hose, then put something heavyish on the hose to keep it in place in the bath and clamp one end with something and then take it to the expansion tank to do the syphoning( can one use one’s thumb? Might be tricky as one negotiates getting into the loft)